New York magazine, as part of their Urban Etiquette Handbook, has a section on “The Four Levels of iPod Interaction : Whom you do and don't have to unplug for” :
LEVEL ONE
Continue at full blast. Consider increasing the vigor of your head-nodding and/or humming.
• Guys passing out bargain-electronics-store flyers.
• Idealistic-looking whippersnappers holding clipboards.
• Scientologists.
